A diazenidoborate: Formation and structure of Cs[(CF3)3BN=NCF3]

Article abstract of DOI:10.1515/znb-2000-3-407

Cs[(CF₃)₃BNH₂] reacts with CF₃NO to form cesium trans-trifluoromethyldiazenido-tris(trifluoromethyl)borate Cs[(CF₃)₃BN=NCF₃] (2), the structure of which has been investigated by single-crystal X-ray diffraction. The diazene nature of the anion is confirmed by the length of the N=N bond, 1.231(6) ?, and the size of the N-N-B angle, 115.5(4)°the B-N bond length being 1.559(7) ?. Compound 2 is hitherto the first diazenidoborate which has been fully characterized.

Full text of DOI:10.1515/znb-2000-3-407

A Diazenidoborate: Formation and Structure of Cs[(CF3)3BN=NCF3]
D. J. Brauer, H. Bürger, Y. Chebude, and G. Pawelke
Anorganische Chemie, FB 9, Universität-GH, D-42097 Wuppertal, Germany
Reprint requests to Prof. Dr. H. Bürger. E-mail: buergerl @uni-Wuppertal.de
Dedicated to Professor Othmar Stelzer on the occasion of his 60th birthday
Z. Naturforsch. 55 b, 269-272 (2000); received December 13, 1999
NMR Data, Boron, Fluorine
Cs[(CF3)3BNH2] reacts with CF3NO to form cesium ?ran5-trifluoromethyldiazenido-tris(tri-
fluoromethyl)borate Cs[(CF3)3BN=NCF3] (2), the structure of which has been investigated by
single-crystal X-ray diffraction. The diazene nature of the anion is confirmed by the length
of the N=N bond, 1.231(6) A, and the size of the N-N-B angle, 115.5(4)° - the B-N bond
length being 1.559(7) A. Compound 2 is hitherto the first diazenidoborate which has been fully
characterized.
in methanol was reacted with a tenfold excess of
Introduction
CF3NO at room temperature. According to 19F
NMR measurements made on the reaction m ix-
ture, 28 % of the aminoborate was converted to
Cs[(CF3)3BN=NCF3] (2) according to eq. (1).
In a preceding paper we have reported on the elec-
trochemical and ozone oxidation of (CF3X3B NH3
(1) [1]. In alkaline solution, first the nitrogen atom
of 1 is deprotonated to form the aminoborate an-
ion [(CF3)3BNH2r , which behaves like a primary
amine. It can be oxidized to yield the nitroborate
[(CF3)3B N 0 2] - , the azodioxy-bis-tris(trifluorome-
thyl)borate [(CF3)3BN(0 )=N(0 )B(CF3)3]2“ , and
the azoxy-bis-tris(trifluoromethyl)borate [(CF3)3-
BN =N (0)B(C F3)3]2“ . The species most closely re-
lated to this series of compounds, the azo-bis-tris-
(trifluoromethyl)borate [(CF3)3BN=NB(CF3)3]2~,
was not obtained, and we have been speculating
about the stability of such an anion. In order to
test the accessibility of the related azoborate unit,
we have attempted to prepare such a species by a
reaction which is well established in organic chem-
istry for the synthesis of azo derivatives F3CN=NR!.
Such compounds are generally formed by the con-
densation reaction of C F3NO with a primary amine
H .N R 1, e.g., CF3NO + H2NCH3 -»• F3CN=NCH3
+ H2O [2]. We wished to test whether we could per-
form the above reaction with [(CF3)3BNH2]_ . Here
we report on our observations.
Cs[(CF3)3BNH2] + CF3NO
-+ Cs[(CF3)3BN=NCF3] + H20
(1)
Still 16% of the Cs[(CF3)3BNH2] had not re-
acted, and two further signals at -62.3 and
-64.2 ppm with relative intensities of 38 and 16%
were present. These arise from hitherto uniden-
tified tris-trifluoromethylboron species. Traces of
Cs[(CF3)3BF] [3] and Cs[(CF3)3BOH] [1] (ca. 1%)
show that the condensation reaction was accompa-
nied by some decomposition. We conclude that the
two latter products are formed from intermediates
because the azoborate 2 is unexpectedly stable. Our
attempts to increase the yield of 2 by varying the pH,
the solvent, and the stoichiometry of the reactants
failed, more by- and decomposition products rather
than 2 being formed. After removal of the methanol
solvent, 2 was isolated by repeated crystallization
from water as an orange-yellow solid. It is soluble
in polar organic solvents like acetone, acetonitrile
and ether.
A thermogravimetric study of compound 2 re-
veals that it decomposes in two distinct steps. First
a loss of weight of 18% is observed between 146 °C
and 250 °C. Second, further loss of weight of 29%
occurs between 300 °C and 400 °C. This degradation
can be interpreted by an idealized equation:
Results and Discussion
Ammine tris(trifluoromethyl)borane (1), which
is very soluble in water, is a weak acid which,
upon treatment with CsOH, forms the stable salt
Cs[(CF3)3BNH2] [1, 3]. A solution of this salt
K
0932-0776/00/0300-0269 $ 06.00 © 2000 Verlag der Zeitschrift für Naturforschung, Tübingen •www.znaturforsch.com
Brought to you by | University of St Andrews Scotland
Authenticated
Download Date | 6/16/15 4:14 PM

270
D. J. B rauer et al. • The D iazenidoborate C s[(C F 3)3B N = N C F3]
FI12A)
Fig. 1. A perspective drawing of 2 employing
20% probability thermal ellipsoids. The loca-
tion of the low-occupancy fluorine atoms at-
tached to the C(4) atom are illustrated with un-
filled circles.
Table 1. Selected bond lengths [A] and angles [°] for
Cs[(CF3)3BN=NCF3]
- Cs[(CF3)3BF]
Cs[(CF3)3BN=NCF3].
- ~^-CF- »C s[BF4]
1.602(8) B-N(l)
1.612(8)
B-C(l)
B-C(2)
B-C(3)
1.559(7)
1.472(8)
1.231(6)
Compound 2 is stable to hydrolysis. When an aque-
ous solution of 2 is irradiated with the 254 nm
line of a mercury lamp, the BN bond is cleaved,
Cs[(CF3)3BOH] [1] being the major reaction prod-
uct.
C(4)-N(2)
1.617(8)
N(l)-N(2)
116.4(4)
104.7(4)
106.0(4)
111.0(5)
N(l)-B-C(l)
N(l)-B-C(2)
N(l)-B-C(3)
C(l)-B-C(2)
110.3(5)
108.0(5)
115.5(4)
110.3(4)
C(l)-B-C(3)
C(2)-B-C(3)
N(2)-N(l)-B
N(l)-N(2)-C(4)
Cs[(CF3)3BN=NCF3] h v/Hl° >
Cs[(CF3)3BOH] + n 2 + HCF3
and (CF3)2C = 0 [4], W hile numerous azobenzene-
borane complexes have been reported [5], reports
on borylated diazenes (azoboranes or azoborates)
are scarce. (C6H5)2B-N=N-C(=0 )-C6H5 has been
obtained by oxidation of the corresponding hy-
drazino derivative [6], and polymeric (C^H s^B-
N=N-B(C6H5)2 was prepared from (CH3)3Si-N=N-
Si(CH3)3 and (CöHs^BCI [7]. However, these
compounds have not been structurally character-
ized. A formally related species is the dianion
[B6H5(NN)B6H5]2- which, however, reveals an NN
stretching vibration at 2065 cm -1 and possesses a
linear BNNB unit. This anion is therefore deemed
a dinitrogen complex [8] rather than an azo deriva-
tive. Thus we can conclude that 2 is the first bo-
ron-substituted diazene derivative which has been
unambiguously characterized.
The UV/Vis spectrum of 2 in water shows a broad
absorption at Amax = 440 nm (e = 10 cm ” 1 m ol-1 )
with a width at half height of ca. 60 nm. The formal
replacement of a methyl group in F3CN=NCH3,
which has Amax = 353 nm (e = 3.58 cm -1 m ol-1 )
[2], by the [B(CF3)3]- moiety causes a significant
red shift of 87 nm and an increase of e.
The l9F, 13C, and "B NMR chemical shifts and
coupling constants of 2 (see Experimental) are con-
sistent with the proposed structure. The relatively
large V(BC) coupling constant of 12.5 Hz should
be mentioned.
According to the infrared and Raman spectra of
2 this novel borate contains a (CF3)3B group, the
typical vibrations of which have been discussed [1].
Of particular interest is the N=N stretching vibration
which gives rise to a medium intensity Raman line
and an IR band at 1595 cm -1 . This is close to the
value reported for F3CN=NCH3 (1592 cm - 1) [2]. A
further strong Raman line observed at 235 cm -1 is
tentatively ascribed to the bending mode £(B-N=N).
The anion of 2 is isoelectronic with the neu-
tral species (CF3)3CN=NCF3, which has been ob-
tained in 2.5% yield by irradiation of CF3CN
Description of the Crystal Structure
A drawing of the anion of 2 is displayed in Fig. 1,
and selected bond lengths and angles are listed in
Table 1. The structure is flawed by the 84:16 disor-
der of the trifluoromethyl group bonded to the N(2)
atom. The minor conformation has its C(4)-F(10A)
bond roughly eclipsing the nitrogen-nitrogen dou-
Brought to you by | University of St Andrews Scotland
Authenticated
Download Date | 6/16/15 4:14 PM

271
D. J. B rauer et al. ■The D iazenidoborate C s[(C F 3)3B N =N C F3]
5, 123(1)°. Perhaps these angular differences re-
flect not only the structural importance of the ni-
trogen lone pair in 2 but also the steric demands of
the formamidine and azodioxy groups. The struc-
ture of the azoborate 2 deviates markedly from
that found for the bridging of two pentahydro-
closo-hexaborate groups by a dinitrogen unit in
[AsPh4]2[B6H5NNB6H5] [8] for which shorter
B-N (1.42(1) A) and N-N (1.129(8) A) bond
lengths and B-N-N bond angles of 180° were
reported.
Nonbonded repulsions are probably responsible
for the large spread in the N (l)-B -C bond angles in
2. The largest angle (116.4(4)°) involves the C (l)
atom, the B-C (l) bond of which roughly eclipses
the diazene linkage (torsion angle -13.9(7)°), and
the other two N (l)-B -C angles are on the average
11(1)° smaller.
CF3\
c f 3— b - n h 3
c f 3/
c f 3\
-B—N
Cs
c f 3-
/
N
CF3
c f 3
c f 3n
N = N
c f 3
Cp3\
CF3— B - N
H
c—NMe2
CF
O
I
c f 3
\
,c f 3
B—N
c f 3-
5
Cs2
N - B -
N -CF3
CF
c f 3
Contacts have been detected between cesium and
fluorine atoms in a variety of cesium salts of triflu-
oromethylborates, and these interactions generally
appear to be weaker than contacts involving cesium
and other nucleophilic sites of the anions [1, 3].
In 2 the trifluoromethyl groups compete with the
diazene group for cesium coordination. Excluding
the fluorine positions of low occupancy, the cation
in 2 enters ten Cs - F approaches in the 3.194(4) to
3.450(13) A range and forms two Cs---N contacts
of 3.405(5) and 3.421(4) A. The relative importance
of Cs •• F interactions in 2 probably reflects the fact
that the fluorine atoms are located on the surface of
the anion whereas the nitrogen atoms occupy less
exposed sites. W hile the nucleophilicity of the triflu-
oromethyl group should be affected by the element
to which it is bonded, we note that no sharp distinc-
tion is found between the eight Cs - F approaches
involving the B-CF3 entities and the two associated
with the N-CF3 fragment. The anion of 2 makes
contacts with five distinct cations. Only those to the
cesium cation in the asymmetric unit are shown in
Fig. 1.
ble bond, and this is the rotam er found by gas phase
electron diffraction for the trilluoromethyl groups
in rra«s-CF3N=NCF3 (3) [9]. However, this confor-
mation is destabilized by packing forces in the solid
state because the F(1 1A )-F(7)(-Jt, 2 - y , 1 - z) and
F(12A) - F(3)(0.5 - x , 0.5 + y, 1.5 - z) separations
of 2.68(3) and 2.85(3) A, respectively, are shorter
than the sum of the corresponding van der Waals
radii of fluorine (2.94 A) [10].
The N (l)-N (2) and N(2)-C(4) bond lengths of
1.231(6) and 1.472(8) A, respectively, do not dif-
fer significantly from the values 1.235(10) and
1.460(6) A reported for these bonds in 3. The N (l)-
N(2)-C(4) bond angle (110.3(4)°) in 2 is slightly
smaller than the 113.4(16)° value found in the gas
phase perhaps because of the above-mentioned dif-
ference in the major conformation of the nitrogen-
bonded trifluoromethyl group in 2 and 3.
It is interesting to compare the B -N (l) bond
length in 2, 1.559(7) A, with the bonds of tetracoor-
dinate boron to nitrogen in other multiply-bonded
systems. The bond in 2 is slightly but not signifi-
cantly longer than that determined for the formami-
dine adduct (CF3)3BNHCHNM e2 (4), 1.541(4) Ä
[11], marginally shorter than the bonds to the azo-
dioxy groups^ in Cs2[(CF3)3BN(0 )N(0 )B(CF3)3]
(5), 1.59(2) A [1], and significantly shorter than
the linkage to the nitro group in Cs[(CF3)3BNC>2],
1.606(4) A [1], The B-N (l)-N (2) bond angle in
2, 115.5(4)°, is markedly smaller than the B-N-
Experimental
NMR: Bruker ARX 400 (100.6 MHz and 376.5 MHz
for 13C and 19F respectively), Bruker AC 250 (79.8 MHz
for11B). [Dö]acetone as solvent and internal standard 13C:
<$c = 30.5, 19F: external standard CFCI3 , n B: external
Standard BF3 OEt2. - IR: Bruker IFS 25; - Raman: Cary
82, Kr+ 647.1 nm. - Thermogravimetry: Netzsch STA
C angle in 4, 127.3(3)°, and the B -N -0 angle in 409 - UV/Vis: Shimadzu UV-2102PC.
Brought to you by | University of St Andrews Scotland
Authenticated
Download Date | 6/16/15 4:14 PM

272
D. J. Brauer et al. ■The Diazenidoborate Cs[(CF3)3BN=NCF3]
Table 2. Crystallographic data for Cs[(CF3)3BN=NCF3],
(BCF3) -63.5 ppm, 2/(BF) 25.2 Hz; 6 (NCF3) -78.2. UB
NMR: 6 (B) -9 .5 ,27(BF) 25.2 Hz. ,3C NMR: 6 (BCF3)
133.2, 'y(BC) 77 Hz, '/(CF) 307 Hz; 6 (NCF3) 119.8,
‘./(CF) 276.7 Hz, V(BC) 12.5 Hz. - UV/Vis H20: Amax =
440 nm, e = 10 cm-1 m oP1- IR (film) / Raman cm-1:
1595 m/1591 m i/(N=N); 1278 vs/1275 m, 1194 vs/-,
1116 vs/1112 m, 1005 s/1005 w i/(CF3); 693 s/-, -/722 vs
<5s(CF3); -/309 s, -/282 s p(CF3); -/235 s <$(B-N=N).
A crystal of 2 was glued to a glass fiber and transferred
to a Siemens P3 diffractometer which employed gra-
phite-monochromated MoKQ radiation. Intensities were
derived from the profiles of the 9/29 scans and were cor-
rected for Lorentz and polarization effects, the drift of
the three periodically-monitored standard reflections and
absorption by integration. The structure was solved by
direct methods. Following anisotropic refinement, max-
ima in a difference Fourier map indicated an alternative
conformation of the trifluoromethyl group of the C(4)
atom. The anisotropically refined atoms F(10), F( 11) and
F(12) were assigned an occupancy a, and the atoms of
the alternative conformation were given occupancies of
1 - q and were refined isotropically. The value of a
converged to 0.84(1), and the C(4)-F bond lengths of
the conformation of higher occupancy were determined
more precisely (1.27(1) A) than those of the other rotamer
(1.28(15) A). Crystal data are summarized in Table 2.
Structural solution, refinement and graphical display were
all made with a SHELXTL program package [12]. Crys-
tallographic data for 2 have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication no. CCDC-137707. Copies of the data can
be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [Fax: int. code
+44(1223)336-033, E-mail: deposit@ccdc.cam.ac.uk].
Empirical formula
Formula weight
Crystal system
Space group
a (A)
b (A)
c (A)
C4BCsF,2N,
447.78
Monoclinic
P2( 1)/n
7.6764(11)
11.543(2)
13.386(2)
99.169(12)
1170.9(3)
4
ßCl
V (A )
Z
2.540
MoKQ
Dcalc (g cm-3)
Radiation
Wavelength (A)
T(K)
Diffractometer
Scan mode
9 Range (°)
Limiting indices
0.71073
294(2)
Siemens P3
Profile data from 9/26 scans
2.34 to 25.05
-9 < h < 9 ,-\3 < k < 13,
-15 < / < 15
6366
Reflections collected
Unique
2070
R(int)
0.0495
Observed (I > 2<r)
Crystal size (mm)
H(mm-1)
Transmission
FI (all data)
1587
0.44x0.10x0.05
3.310
0.8499 to 0.7175
0.0449
wR2 (all data)
Goodness-of-fit on F2
Parameters
0.0867
0.997
194
zAF map (e A-3)
0.802 to -0.891
Cesium trans-trifluoromethyldiazenido-tris(trifluoro-
methyl)borate Cs[(CF3)^BN=NCF^] (2): Onto a solution
of 2 g (5.5 mmol) of Cs[(CF3)3BNH2] in 5 ml of abs.
methanol 50 mmol of CF3NO were condensed at -196 °C
and stirred for 48 h at 20 °C. Unreacted CF3NO and
methanol were removed in vacuo, and the residue was
crystallized from water. Yield 0.3 g (12%). - l9F NMR: 6
Acknowledgements
Financial support by the DAAD to Y.C. and by the Fonds
der Chemischen Industrie is gratefully acknowledged.
[1] D. J. Brauer, H. Bürger, Y. Chebude, G. Pawelke,
Eur. J. Inorg. Chem. 247 (1999).
[7] N. Wiberg, G. Schwenk, Angew. Chem. 81, 745
(1969).
[2] A. H. Dinwoodie, R. N. Haszeldine, J. Chem. Soc.
2266 (1965) and Russian literature cited therein.
[3] D. J. Brauer, H. Bürger, Y. Chebude, G. Pawelke,
Inorg. Chem. 38, 3972 (1999).
[8] A. Franken, W. Preetz, Z. Naturforsch. 50b, 781
(1995).
[9] H. Bürger, G. Pawelke, H. Oberhammer, J. Mol.
Struct. 84, 49 (1982).
[4] R. S. Kirchmeier, U. I. Lasouris, J. M. Shreeve, Inorg.
Chem. 14, 592 (1975).
[10] A. Bondi, J. Phys. Chem. 68, 441 (1964).
[11] A. Ansorge, D. J. Brauer, H. Bürger, B. Krumm,
G. Pawelke, J. Organomet. Chem. 446, 25 (1993).
[12] G. M. Sheldrick, SHELXTL PC Version 5.03: An
Integrated System for Solving, Refining and Dis-
playing Crystal Structures from Diffraction Data;
Siemens Analytical X-ray Instruments Inc. (1994).
[5] R. Köster in Houben-Weyl, Methoden der Organ-
ischen Chemie: Organoborverbindungen II, 133/3b,
Georg Thieme, Stuttgart/New York (1983).
[6] H. Nöth, W. Regnet, H. Rihl, R. Standfest, Chem.
Ber. 104. 722 (1971).
Brought to you by | University of St Andrews Scotland
Authenticated
Download Date | 6/16/15 4:14 PM